I attended and spoke at the very first UK Open Data Camp. This weekend event was in Winchester. Unfortunately, the company didn't want to pay for my hotel so I coughed up myself. It was well worth it as I met lots of important Open Data contacts and the sessions were mostly very interesting.
I got a good sneak peek at the Ordnance Survey's open water layer. They say that it isn't as detailed as MasterMap so might be no use to us.